The present invention relates to semiconductor design technologies; and, more particularly, to a semiconductor memory device which is provided with a refresh device capable of minimizing power consumption by adjusting a refresh period depending on a level of an internal power.
Generally, basic operations of a semiconductor memory device include a write operation for storing data provided from outside and a read operation for outputting desired data stored therein upon request. In order to perform these write and read operations, the semiconductor memory device requires the capability of storing data provided from outside.
In particular, since DRAM (Dynamic Random Access Memory) is a nonvolatile memory, data stored therein is lost after a fixed amount of time elapses. Therefore, a refresh operation is needed to fully restore data stored in memory cells before the fixed amount of time in order to prevent the loss of data. This refresh operation has priority over any other operations within DRAM.
Such a refresh operation is done at a fixed period that is closely concerned with a retention time of cell data. More details of this will be given below with reference to a cross-sectional view of unit memory cell.
FIG. 1 is a cross-sectional view of unit memory cell, and particularly shows a leakage current in the unit memory cell.
As shown in FIG. 1, although a transistor is turned off, a leakage current occurs in a cell capacitor (here, it is assumed that logic high data is stored in the cell capacitor). Two major factors that cause the leakage current are an off current IOFF and a junction current IJUN.
Meanwhile, an internal power VBB with a negative electric potential lower than a ground voltage is generally connected to a bulk of an NMOS transistor in unit memory cell shown in FIG. 1. By keeping the bulk bias low like this, the leakage current is reduced by setting a threshold voltage of transistor higher than that of a general NMOS. But, as a level of the internal power VBB lowers, the threshold voltage becomes higher to decrease off-current, whereas the leakage current increases by increase of junction-current. That is, the off-current and junction-current constituting the leakage current have a trade-off relationship depending on the level of the internal power VBB. Therefore, in order to lengthen a retention time of cell data, it is important to find the level of an optimal internal power VBB making both the off-current and junction-current smaller.
Now, a refresh device for refreshing cell data will be described.
FIG. 2 is a block diagram showing a configuration of a semiconductor memory device having a conventional refresh signal generator.
Referring to FIG. 2, the conventional semiconductor memory device includes a mode input/output controller 10 for generating an internal auto refresh signal AREFP, a self refresh entrance signal SREF_EN and a self refresh escape signal SREF_EXP based on a clock enable signal CKE and an auto refresh command AREF_CMD, a refresh interval signal generator 20 for generating a self refresh interval signal SREF notifying a self refresh interval by using the internal auto refresh signal AREFP, the self refresh entrance signal SREF_EN and the self refresh escape signal SREF_EXP, a refresh period signal generator 30 for periodically outputting a period-pulse signal PL_FLG during activation of the self refresh interval signal SREF, an internal refresh signal generator 40 for activating an internal refresh signal REFP in response to the internal auto refresh signal AREFP and the period-pulse signal PL_FLG, and an internal address counter 50 for increasing a row address by one bit unit in response to the internal refresh signal REFP to output an internal address RCNTI[0:N].
For reference, the clock enable signal CKE is a signal indicating whether a clock synchronizing the operation of a semiconductor memory device is valid or not. Thus, if only the clock enable signal CKE is inactivated, the semiconductor memory device enters a power-down mode for minimizing its own power consumption.
FIG. 3 shows an internal circuit diagram of the refresh period signal generator 30 of FIG. 2.
Referring to FIG. 3, the refresh period signal generator 30 includes a driving power supplier 32 for voltage-dividing a power supply voltage VDD and a ground voltage VSS to provide divided voltages as driving powers, an oscillator 34 which has an inverter chain and is active during activation of the self refresh interval signal SREF to generate a signal OSC_OUT at regular intervals depending on levels of the driving powers, and a pulse output circuit 36 for producing the output signal OSC_OUT of the oscillator 34 as the period-pulse signal PL_FLG of pulse type.
In brief operation, first of all, when the self refresh interval signal SREF is activated to a logic high level, the oscillator 34 generates the signal OSC_OUT at regular intervals. Here, the regular intervals are determined based on the voltage levels of the driving powers applied to the gate ends of NMOS transistors and PMOS transistors forming the inverter chain therein. As mentioned above, the driving powers are provided by voltage-dividing the power supply voltage VDD and the ground voltage VSS by the driving power supplier 32. Next, the pulse output circuit 36 senses a rising edge of the output signal OSC_OUT of the oscillator 32 to generate the period-pulse signal PL_FLG of pulse type.
Now, the operation of the semiconductor memory device having the conventional refresh signal generator shown in FIGS. 2 and 3 will be briefly described.
First, when the clock enable signal CKE is transited to a logic low level, the auto refresh command AREF_CMD is activated. Then, the mode input/output controller 10, in response to the transition of logic level of the clock enable signal CKE, activates the self refresh entrance signal SREF_EN, and, in response to an auto refresh command AR, activates the internal auto refresh signal AREFP.
Next, the internal refresh signal generator 40 generates the internal refresh signal REFP in response to the internal auto refresh signal AREFP. In succession, the internal address counter 50 increases the row address by one bit unit whenever the internal refresh signal REFP is activated, to output the internal address RCNTI[0:N].
Further, the refresh interval signal generator 20 activates the self refresh interval signal SREF in response to activation of the internal auto refresh signal AREFP and the self refresh entrance signal SREF_EN, wherein this activation is maintained until the self refresh escape signal SREF_EXP is applied.
Next, the refresh period signal generator 30 periodically activates the period-pulse signal PL_FLG during the activation of the self refresh interval signal SREF. And then, the internal refresh signal generator 40 activates a new internal refresh signal REFP of pulse type whenever the period-pulse signal PL_FLG is applied. The internal address generator 50 increases the row address by one bit unit whenever the internal refresh signal REFP is activated, to output the internal address RCNTI[0:N].
For reference, the internal refresh signal REFP is applied to each band, so that a word line corresponding to the internal address RCNTI[0:N] becomes active to perform self refresh.
Meanwhile, the refresh period by the refresh device within the conventional semiconductor memory device is determined by the period of the period-pulse signal PL_FLG. The period-pulse signal PL_FLG is generated to have a constant period regardless of the level of the internal power VBB. Therefore, although the level of the internal power VBB is optimized so that the retention time is reduced, it is not likely to reflect the above. As a result, this reduces the number of times of refresh and thus cannot reduce power consumption.